The H 2O 2-based epoxidation of bridged cyclic alkenes in a monophasic system containing low concentrations (<2 mM) of [Bu 4 n N] 4[Pr 2 i NH 3] 2H[P{Ti(O 2)} 2W 10O 38]·H 2O ( 1) (with two η 2-peroxotitanium sites in the anion) has been studied in search of the catalytically active species involved. 31P NMR spectra of 1, measured under a variety of conditions, revealed that the active species was not hydroperoxotitanium complex [P{Ti(OOH)} 2W 10O 38] 7−or [P{Ti(OOH)}Ti(O 2)W 10O 38] 7−. The reaction pathways for the alkene epoxidation are discussed to understand the kinetics (especially the initial [H 2O 2] dependence). It was concluded that the net catalytic reaction for the epoxidation occurred through the two-electron oxidation at the hydroperoxotitanium site in the catalyst.